Merge pull request #499 from CommunityToolkit/dev/nullable-get-ref

Use Nullable.GetValueRefOrDefaultRef where possible

CommunityToolkit.HighPerformance/Extensions/NullableExtensions.cs

@@ -35,7 +35,11 @@ public static class NullableExtensions
     public static ref T DangerousGetValueOrDefaultReference<T>(this ref T? value)
         where T : struct
     {
+#if NET7_0_OR_GREATER
+        return ref Unsafe.AsRef(in Nullable.GetValueRefOrDefaultRef(in value));
+#else
         return ref Unsafe.As<T?, RawNullableData<T>>(ref value).Value;
+#endif
     }
 
     /// <summary>
@@ -46,17 +50,41 @@ public static ref T DangerousGetValueOrDefaultReference<T>(this ref T? value)
     /// <returns>A reference to the value of the input <see cref="Nullable{T}"/> instance, or a <see langword="null"/> <typeparamref name="T"/> reference.</returns>
     /// <remarks>The returned reference can be tested for <see langword="null"/> using <see cref="Unsafe.IsNullRef{T}(ref T)"/>.</remarks>
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    public static unsafe ref T DangerousGetValueOrNullReference<T>(ref this T? value)
+    public static ref T DangerousGetValueOrNullReference<T>(ref this T? value)
         where T : struct
     {
+#if NET7_0_OR_GREATER
+        ref T resultRef = ref Unsafe.NullRef<T>();
+
+        // This pattern ensures that the resulting code ends up having a single return, and a single
+        // forward branch (the one where the value is null) that is predicted non taken. That is,
+        // the initial null ref is very cheap as it's just clearing a register, and the rest of the
+        // code is a single assignment (lea on x86-64) that should always be taken. This results in:
+        // =============================
+        // L0000: xor eax, eax
+        // L0002: cmp byte ptr[rcx], 0
+        // L0005: je short L000b
+        // L0007: lea rax, [rcx + 4]
+        // L000b: ret
+        // =============================
+        // This is better than what the code would've been with two separate returns in the method.
+        if (value.HasValue)
+        {
+            resultRef = ref Unsafe.AsRef(in Nullable.GetValueRefOrDefaultRef(in value));
+        }
+
+        return ref resultRef;
+#else
         if (value.HasValue)
         {
             return ref Unsafe.As<T?, RawNullableData<T>>(ref value).Value;
         }
 
         return ref Unsafe.NullRef<T>();
+#endif
     }
 
+#if !NET7_0_OR_GREATER
     /// <summary>
     /// Mapping type that reflects the internal layout of the <see cref="Nullable{T}"/> type.
     /// See https://github.com/dotnet/runtime/blob/master/src/libraries/System.Private.CoreLib/src/System/Nullable.cs.
@@ -70,6 +98,7 @@ private struct RawNullableData<T>
         public T Value;
 #pragma warning restore CS0649
     }
+#endif
 }
 
 #endif

azure-pipelines.yml

@@ -37,7 +37,6 @@ jobs:
     displayName: Install the .NET 7 SDK
     inputs:
       version: $(DotNet.Version)
-      includePreviewVersions: true
       performMultiLevelLookup: true
 
   # Set Build Version